a) Field of the Invention
The present invention relates to a novel core-shell synthetic resin emulsion, to coating compositions for paints paper coating and information recording papers, and based thereon, and to a process for its production.
b) Description of the Related Art
In recent years, various kinds of high molecular weight polymer particles have been studied for use as additives, such as pigments and fillers, for coating compositions.
The most commonly used polymer particularized are homogeneous and solid, emulsion-polymerized polystyrene particles having a diameter of 0.2 to 0.5.mu.. For example, Japanese Patent Laid-Open No. 59741/'84 discloses a process wherein an unsaturated carboxylic acid and a vinyl monomer are copolymerized in the presence of an anionic surface active agent and/or a nonionic surface active agent to form a copolymer emulsion in which more than 90% of the particles have a diameter of 0.20 to 0.28.mu.. It is also described therein that the copolymer emulsion can be used as an organic pigment in applications such as paper coating and paints. However, the thus-produced organic pigment fails to provide sufficient hiding power, brightness and gloss and no practical advantage is obtained unless it is used in a large amount.
In order to improve hiding power, brightness and gloss of coating composition based on emulsion polymerized polymer particles, the use of particles having pores, unlike the above homogeneous and solid particle, has recently been proposed in U.S. Pat. No. 4427836. That patent discloses a process for producing an aqueous dispersion which comprises providing a dispersion of core particles formed from a polymer containing at least 5% of a carboxylic acid monomer, adding thereto at least one monoethylenically unsaturated sheath monomer for forming a sheath polymer on the core particles subjecting this monomer to emulsion polymerization, and neutralizing the resulting emulsion with an aqueous volatile base to swell the core particles and thereby form minute openings therein.
When the polymer particles produced by the above patented process are used as an organic pigment in paints or paper coating compositions, an improvement in hiding power and brightness is achieved as compared with the use of organic pigments based on homogeneous solid polymer particles. However, in order to obtain sufficient minute openings in the particles to improve hiding power and brightness by the process, a large amount of unsaturated carboxylic acid must be employed to produce the core particles. As a result, the hydrophilic property of the core particles is enhanced and it becomes difficult to uniformly coat the core particles with the sheath polymer, which leads to the formation of secondary particles which have poor properties. Further, in view of the mechanism that generates the minute openings with neutralization-swelling of the core particles containing carboxyl groups, it is also difficult to control the size of the minute openings for specific end-use applications.
Japanese Patent Laid-Open No. 62510/'86 discloses a process for the production of a synthetic resin emulsion which forms emulsion particles with pores in the interior thereof by utilizing the phenomena of phase separation between different kinds of polymer particles in the polymerization reaction and of volumetric shrinkage resulting from the polymerization. However, an organic pigment based on the polymer particles does not have improved properties, e.g., increased gloss, hiding power and brightness because the pores formed in the particles are too minor.